The field of the invention is that of headboxes for a paper machine.
A headbox for dispensation of a suspension, consisting of fibers, filler and fine substances as well as chemical agents and water, on a running wire is for producing a paper web. The suspension stream dispensed by the headbox must have a constant thickness, equal velocity, equal stream direction and equal consistency (density) across a machine width. This produces a paper web with consistent properties across the machine width, specifically a consistent basis weight cross profile. Variations in the basis weight cross profile are sought to be eliminated in the following way.
A headbox for a paper machine is described in West German patent No. 2,912,152. Diluting liquid is supplied to the machine wide suspension channel in a quantity that is adjustable across the machine width. This compensates for variations in the basis weight of the finished web by influencing the density of the suspension. To that end, the diluting liquid is introduced in the suspension flow, the diluting liquid preferably being tail water that mixes well with the suspension. It is introduced in an area that is located between a cross distributor for the suspension supply and a turbulence generator of the headbox. Alternatively, the diluting liquid can be introduced either directly in the turbulence generator or in the outlet of the headbox coordinated with a cylinder.
This has a grave disadvantage due to the location and the way of feeding diluting liquid into the suspension flow. Because of those factors, there are hydraulic disturbances caused in the suspension flow. The effects of these hydraulic disturbances are greater in width the longer the travel of suspension from the admixture of the diluting liquid, up to the fixing of the fiber on the wire. Since the prior headbox does not feature partitions that guide the suspension flow, the disturbance can propagate unimpededly. An addition of diluting liquid that varies from the remaining area of the headbox causes cross flows which the turbulence generator, fashioned as a perforated roll, is unable to be eliminated. The result, therefore, is an unfavorable fiber orientation in adjoining areas of the paper web which a correction of the basis weight cross profile is not intended. Also, feeding the diluting liquid transverse to the suspension flow direction causes coarse turbulences in the suspension flow. These coarse turbulences can no longer be compensated for in the headbox and, therefore, lead to formation problems in the finished paper web.
Therefore, what is needed is a headbox for a paper machine able to influence the basis weight cross profile locally in a closely limited area without causing disturbances in the remaining web area.